Analysis of blueshifted emission peaks in Type II supernovae

In classical P-Cygni profiles, theory predicts emission to p eak at zero rest velocity. However, supernova spectra exhibit emission that is generally blue shifted. While this characteristic has been reported in many supernovae, it is rarely discussed in any detail. Here we present an analysis of Hα emission-peaks using a dataset of 95 type II supernovae, quantifying their strength and time evolution. Using a post-explosion time of 30 d, we observe a systematic blueshift of Hα emission, with a mean value of ‐2000 km s 1 . This offset is greatest at early times but vanishes as supernovae become nebular. Simulations of Dessart et al. (2013) match the observed behaviour, reproducing both its strength and evolution in time. Such blueshifts are a fundamental feature of supernova spectra as they are intimately tied to the density distribution of ejecta, which falls more rapidly than in stellar wi nds. This steeper density structure causes line emission/absorption to be much more confined; it also exacerbates the occultation of the receding part of the ejecta, biasing line emission to t he blue for a distant observer. We conclude that blue-shifted emission-peak offsets of sever al thousand km s 1 are a generic property of observations, confirmed by models, of photosphe ric-phase type II supernovae.